Smoke detection devices have been introduced or proposed to the market place in a wide variety of configurations ranging from the simple and crude to highly sophisticated and concomitantly expensive systems intended for industrial or military applications.
The consuming public is now becoming aware of the value of some form of fire warning system for the home. For acceptance into this popular market, such devices must be made available in large volume at reasonable prices, yet should retain a technical sophistication assuring as high a sensitivity as possible, and, of particular importance, an assured extended term reliability.
Improved sensing techniques now provide design opportunity to achieve a relatively high rate of response to smoke or related combustion products. For instance, those sensing techniques in general use may be photo-optical of a variety wherein the occlusion of a direct beam of select radiation is detected; or through utilization of the Tyndall effect, responding to particle reflectance. Additionally, ionization devices are available for combustion product detection.
While such detection schemes are known, a fire protection arrangement incorporating one of the above-cataloged sensors must also meet somewhat extensive consumer criteria. For example, the entire detection and alarm system should be packaged as a relatively small, convenient unit readily mountable while remaining unobtrusive at such locations within a household as the ceiling or upper wall of a selected room or stairwell. The distaff element of most households as well as an increasing number of governmental building codes show preference or require alarm units which are not line energized, i.e., which incorporate battery power supplies while still retaining compact overall configurations.
To the present, however, conventional, readily available batteries have been found to exhibit output characteristics unsuited to the demands of compact fire alarm units. Accordingly, specialized power supplies not readily available and expensive have been required for operating the detector-alarm units. Generally, the operating lifespan for these batteries is limited to a one-year duration.
Another performance aspect prevalent in certain existing smoke detection systems resides in the exhibition of a diminishing acuity or responsiveness over their operational lifespan. This undesirable attribute obtains in both line as well as battery powered systems. In certain instances, rather elaborate adjustment procedures are required of the operator following suggested intervals of operation to accommodate for sensitivity fall-off. Where battery power supplies are incorporated, sensing acuity generally diminishes in correspondence with the lessening of battery output. Of course, the reliability of all such systems is subject to question where any significant form of attendance to adjustment detail is required on the part of the lay public.
From the foregoing, it may be observed that what would be considered most desirable attributes for smoke detectors suited for the noted broadened consumer market would be a compact, battery powered unit which may remain unattended over relatively extended periods of time. Further, such a unit should be powered by small universally available and popularly priced batteries. Additionally, it is desirable that the alarm units be capable of generating a positive, perceptive signal indicating an active ongoing surveillance condition. From the standpoint of reliability, the sensitivity of the smoke detection units should not diminish with corresponding fall-off of available battery derived power.
In addition to the above criteria, the detector-alarm units must be structured to perform reliably under relatively extreme environmental conditions as required by national testing organizations. For instance, such conditions include a necessary circuit stability over broad ranges of temperature. While circuit techniques utilizing, for example, silicon components suggest that low power consumption detector systems might be designed, the sensitivity of such components to temperature excursions heretofore has blocked their implementation within practical smoke detector designs.